The present invention relates to the general field of so-called “passive” acoustic treatment.
The application domain of the invention is particularly, but not exclusively, that of aviation turbine engines.
The use of passive acoustic treatment panels is frequent in the field of aviation in order to reduce the emission of sound from turbine engines. For example, with a two-spool bypass type turbojet, such panels may be arranged on the inside face of the nacelle that surrounds the turbojet, and also on the outside face of its primary cover.
A passive acoustic treatment panel is generally constituted by a honeycomb cellular structure covered by a thin porous layer acting as a wall for a gas flow passage and closed on its other side by a layer that is reflective from an acoustic point of view (also referred to as a continuous skin). More complex panels with intermediate porous layers inserted between honeycomb structure layers may be provided in order to extend attenuation over a broader frequency range. The present invention applies more particularly to such an acoustic treatment panel known as a “multilayer” panel.
It is known that sound attenuation is much more effective when the helical acoustic modes on which sound energy propagates are of high order. These acoustic modes remain in the ducts for longer and they present energy closer to the wall, thereby facilitating dissipation of their energy by panels placed on the walls of the nacelle or of the turbojet. In addition, the sound attenuation of an acoustic treatment panel varies in relatively linear manner up to a certain treatment length, and beyond that length sound attenuation increases much more slowly. Such behavior may be explained by the variation in the characteristics of the source sound that, as it propagates through the panel, becomes less and less rich in high-order acoustic modes. Since sound attenuation depends on which acoustic modes are present, the presence of a larger number of low-order acoustic modes for transporting the remaining acoustic energy implies that the absorption efficiency of the panels is smaller. Unfortunately, presently-used acoustic treatment panels do not enable these two requirements to be satisfied.